Conductive plastic compositions have been well received as desirable raw materials for the fabrication of a variety of specialized accessories and components including static electricity dissipation devices, electrical heating elements, equipment parts for high frequency protection and/or electromagnetic interference (EMI) shielding, and a variety of other electrical components such as electrodes, terminals, connectors and the like.
Thermosetting or heat-curable polymer systems have been most prominent in the majority of such conductive plastics materials which have been developed so far. For certain electrical applications, the resistance of many thermosetting materials to high temperature service conditions is a major consideration. However, a more generally important factor probably resides in the inherent reactivity responsible for their thermosetting character which also tends to encourage polymeric interaction with finely subdivided, conductive solids (e.g., metallic powders, carbon blacks and the like) that are incorporated into the polymeric base material in order to provide appropriate levels of conductivity.
Most thermoplastic resins on the other hand are considerably less responsive to additions of finely divided solid fillers so that actual deterioration of many structurally significant physical properties usually occurs when they are filled with carbon blacks, powdered metals and the like in amounts required to provide practical levels of electroconductivity. Such deficiencies have severely limited applications accessed by conductive thermoplastic compositions, largely confining them thus far to the fabrication of partly supported auxiliary elements or secondary components like seals, gaskets, inserts and electrodes, rather than primary structural members.
In spite of such difficulties, filled thermoplastic systems have, of course, continued to receive considerable attention since rigid thermoplastic resins offer definite advantages over most thermosetting materials in regard to ease of handling, melt processing convenience and simplicity of fabricating finished articles therefrom by usual high speed plastic forming techniques such as extrusion, molding and the like. Indicative of approaches which have been taken in an effort to develop metal-filled thermoplastic compositions with improved overall performance and utility are those disclosed in the publications summarized below.
U.S. Pat. No. 3,491,056 to Saunders et al discloses the rare ability of finely divided aluminum powder to strengthen a specialty thermoplastic resin derived from a prescribed copolymerization of ethylene with an unsaturated carboxylic acid such as acrylic acid. It appears, however, that outstanding levels of electrical conductivity were not achieved in this system even with a 50% by volume loading of conductive filler unless some of the fine aluminum powder was replaced with carbon black (e.g., 16% by volume as in Example 7).
U.S. Pat. No. 3,867,315 to Tigner et al is much more concerned with achieving good electrical conductivity levels without excessive volume loadings of the particular metallic filler material. This is accomplished by including various ionic metal salts along with the metallic filler, which is either copper or contains accessible copper. A broad list of thermoplastic resins is recited, but experimental data is presented only for a blend of 2 parts polyethylene with 1 part of 72/28 copolymer of ethylene and vinyl acetate and no physical strength properties whatsoever are indicated. A closely related patent is U.S. Pat. No. 3,919,122 to Tigner which deals with substantially the same system except that the ionic salt is a metal halide salt which is formed in situ from free metal and a suitable halide source. The preferred halide source is a halogen-containing polymer (notably one derived from vinylidene chloride), with a copolymer of vinyl chloride and vinylidene chloride in respective weight proportions of 27:73 being used in most of the illustrative examples. However, the only metallic filler used in said examples is a brass powder with an average particle size of 5 to 12 microns and, again, no physical strength measurements are presented.
Another approach to achieving highly conductive metal-filled plastic composites at very low volume loadings of the metallic filler has been resorted to from time to time in this art. The basis of this approach, which is often referred to as the "segregated metal particle network" technique, is the careful observance of several critical processing conditions in fashioning the finished composite. These conditions generally include dry mixing of rather large granules of organic polymer with much smaller particles of metal and compacting the resulting mixture under pressures and temperatures controlled to cause some coalescence or sintering between neighboring polymeric granules without effecting sufficient melt flow to result in extensive intermingling with the network of fine metallic particles distributed therebetween. By means of such techniques, highly conductive, compacted metal-polymer composites can be obtained at metal filler loadings below about 10% by volume, due to the resulting preferential segregation of metal particles into extended chain-like networks which apparently serve as a system of three-dimensionally interconnected pathways through which current can flow. Patents describing products made by such techniques include U.S. Pat. No. 2,761,845 to Coler and U.S. Pat. No. 3,708,387 to Turner et al. Additional descriptions are also found in the basic research literature including such recent journal articles as:
Journal of Applied Polymer Science 20, pp. 2575-2580 (1976) by Mukhopadhyay et al and Polymer Engineering and Science 19, pp. 533-544 (1979) by Bhattacharyya et al.
Unfortunately, industrial applications for said products appear to be extremely limited since the associated techniques are totally abhorrent to the high speed, "molten state" mixing and molding operations for which thermoplastic are so well suited and for which reason they are usually selected in commercial practice. Furthermore, in view of the inherent heterogeneous nature of such "segregated network" metal-polymer compacts, it is very doubtful in any case that adequate manufacturing uniformity and reproducibility could be achieved for commercial articles except possibly those of the simplest shape and design and least demanding fields of application.
In view of the apparent state of this art, a considerable need continues to exist for improved and more versatile metal-filled polymeric compositions. In particular, a clear need is sensed for such compositions which are not only derived from a thermoplastic resinous matrix but which can also be economically and conveniently prepared and safely processed and dependably fabricated by conventional high speed techniques into a wide variety of shaped articles having both good conductivity and sound physical integrity. One of the most challenging raw material requirements in this field resides in the need for conductive thermoplastic molding compounds suitable for forming flame retardant structural members of sufficient size, mass and complexity to serve as electronic equipment housings, dampers and/or shields for absorbing or blocking out electromagnetic field effects or other high frequency electrical emissions.
Accordingly, the primary goal of the present invention is the provision of a family of flame retardant, thermoplastic molding compounds of high electroconductivity which can be readily shaped even by fast cycle molding techniques to form rigid articles having well balanced all around physical properties and adequate structural stability for many diversified electric conducting specialty applications. A more specific objective of my invention is to formulate flame retardant, thermoplastic molding compounds the ingredients and composition of which are further restricted and optimized so that exceptional levels of electroconductivity as well as outstanding overall physical properties are attained in articles molded therefrom. Such optimized molding compounds are particularly needed for certain specialized structural uses, such as EMI shielding members, electronic equipment housings and the like, and thus represent a preferred embodiment of the present invention.